One of the things on my ham radio bucket list was to learn how to use KiCAD and design a PC board. Being a newbie at PC board design (when I was an engineer, we had PC designers do the actual layout), I decided to do something simple. Since I teach ham radio classes, I decided to design a board based on the circuit in Figure T-1 of the Technician Class question pool.
This diagram illustrates a couple of ideas:
- How schematic diagrams represent electronic circuits.
- How a transistor is used as a switch.
Designing the board
Since I already had the schematic, the first step in designing the board was to choose the components. I had a bunch of 2N2222 transistors in my junkbox, so that was an easy choice.
Choosing the lamp wasn’t quite so easy, though. Normally, I’d just use an LED, but LEDs require current-limiting resistors, and adding a resistor to the circuit would meant that it wouldn’t match the diagram. I did an Amazon search and found some LEDs with built-in current-limiting resistors.
Since they’re designed to operate from 9 – 12 V, it was natural to choose a 9 V battery. This meant that I had to find the snaps to plug the battery into. A quick Mouser search yielded the Keystone 593 and 594.
Once I’d decided on using a 9 V battery, the next step was to choose the resistor. I figured that connecting the input to the 9 V battery would be the easiest way to turn on the lamp, and I calculated that 47 kΩ resistor would give enough base current to turn on the lamp.
Since the current through the resistor was so low, I thought that an 1/8 W resistor would do. While that’s true, choosing that value probably wasn’t a good idea. The reason for this is that 1/4 W resistors are much more readily available than 1/8 W resistors. The next revision of this board is going to have space for a 1/4 W resistor.
Making the schematic was pretty straightforward, but laying out the board was a little more complicated. First, you have to assign a footprint for each of the components, then place them on the board, then route traces. That doesn’t seem like a big deal, but it takes some time to get used to how to actually do this.
Once I thought I had a decent design, I ran it by one of my friends here in Ann Arbor who has designed several PCBs. His first suggestion was to increase the size of the traces. I had just used the default size, but he noted that it’s easy to lift traces when soldering if they are too small.
He also suggested that I make the board as small as possible to reduce the cost. I did do that, but as you can see from the photo below, the batter hangs off the board. That’s not really a big deal, but I did want to include an image of the Figure T-1 schematic, as the circuit is supposed to demonstrate how the circuit in Figure T-1 works.
To make the board, I chose AllPCB.Com. They were really very patient with me. For example, when I first submitted the design files, I forgot to include the copper layers!
Want one?
I haven’t used one of these in any of my classes yet, but I think it will make an effective demo. If you’re an instructor, and would like to have one of these boards, I can sell you a kit for $6.
Also, if you have any suggestions on how to improve this board, please feel free to contact me.